1. Field of the Invention
The present invention relates to a multi-mode receiver circuit, and more particularly to a multi-mode receiver set for use in radio equipment which can deal with various kinds of modulation systems, signal formats, and coding systems.
2. Description of the Background Art
For integrated circuit (IC) applications, digital processing circuits fabricated on a semiconductor substrate have been reduced in horizontal and vertical dimensions to meet the high-density integration requirement. Because of this, analog circuits and radio-frequency circuits have relatively been increased in geometrical area and manufacturing cost. For that reason, multi-mode receiver circuits for use in radio equipment, which receive signals by adaptively switching modulation systems or signal formats which deal with a plurality of communication systems are designed to share an analog circuit and a high-frequency circuit with those systems or formats in such a fashion that a demodulator for demodulating a received signal by digital processing has its input port at which the received signal is split according to communication systems to demodulate split signals. In this manner, the entire area and cost of the receiver circuits can be reduced. This concept itself is known as software radio or reconfigurable radio circuit.
One of such multi-mode receiver circuits for use in radio equipment which deals with plural kinds of modulation systems and signal formats has been proposed in Japanese Patent Laid-Open Publication No. 284554/1999, by way of example.
The above Japanese '554 publication discloses a multi-mode radio transmission set, which is provided with a plurality of antennas, a high-frequency signal processing circuit, wave detectors and a signal-intensity measuring circuit, which are adaptively switched with switches so as to establish communications. Particularly, in the multi-mode radio set, high-frequency signals, i.e. radio signals, are received by the antennas to the high-frequency signal processing circuits, and are in turn detected and demodulated with the PHS (Personal Handy-phone System) wave detector and PDC (Personal Digital Cellular) wave detector. In the radio transmission set taught by the Japanese '554 publication, either of the two demodulated signals from the wave detectors is output through the switch to a circuit internal to the radio set according to the signal intensities measured with the signal intensity measuring circuits.
Another Japanese Patent Laid-Open Publication No. 2002-368829 discloses a technique in which an antenna and a high-frequency circuit are shared and, through a switch disposed on the output side of the shared section, are connected to either an ASK (Amplitude Shift Keying) demodulator or a QPSK (Quadrature Phase Shift Keying) demodulator. Particularly, this Japanese '829 publication discloses that the electric power of a received signal from the antenna is measured within the sharing section to determine a modulation system used, and the received signal is modulated by either of the two ASK and QPSK demodulators selected with the switch.
Well, multi-mode receiver circuits for use in radio equipment are required to, under the circumstances in which there are possibly input signals in various modulation systems and signal formats, appropriately deal with a type of high-frequency signal that is input at that time. Generally, high-frequency circuits are high in power consumption, large in occupied volume, and costly. Because of this, it is undesirable that the multi-mode receiver circuit be provided with a plurality of high-frequency circuits dedicated to deal with respective modulation systems and signal formats.
Hence, in multi-mode receiver circuits, it would be useful to share a single high-frequency circuit with various modulation systems and signal formats. However, multi-mode receiver circuits are required to determine the modulation system and signal format of an input high-frequency signal and demodulate the received signal properly to the system and format thus determined. Multi-mode receiver circuits are also desired to compare the results of modulation with one another and output optimum one of the results. That causes multi-mode receiver circuits to be complicated in processing such as comparison of digital signals. Multi-mode receiver circuits therefore have difficulties in circuit scale being increased or processing delay being increased. The difficulties will hereinafter be described in detail.
In the multi-mode receiver circuit disclosed in the aforementioned Japanese '829 publication, an antenna for receiving a high-frequency signal is connected with a high-frequency circuit, which is in turn connected to a plurality of demodulators, e.g. a demodulator for BPSK (Binary Phase Shift Keying) modulation, a demodulator for QPSK modulation, and a demodulator for ASK modulation. The three demodulators corresponding to a plurality of modulation systems share the high-frequency circuit that includes an intermediate-frequency filter and an intermediate-frequency amplifier.
Assume that the intensity of a received signal, which is required for the three demodulators to output a demodulated signal while maintaining intended quality, increases in order of BPSK, QPSK, and ASK demodulators. In the multi-mode receiver circuit, when the intensity of a received signal from the antenna is great, i.e. when the ASK demodulator provides the signal intensity of a received signal such that it can output a demodulated signal while maintaining intended quality, each of the three demodulators can output a demodulated signal while maintaining intended communication quality regardless of which of BPSK, QPSK and ASK modulations the modulation system of the received signal is.
On the other hand, when the intensity of a received signal from the antenna is small, i.e. when only the BPSK demodulator provides the intensity of a received signal such that it can output a demodulated signal while maintaining intended quality, if the modulation system of the received signal is BPSK modulation, the BPSK demodulator can output a demodulated signal while maintaining intended communication quality. The QPSK demodulator or ASK demodulator, however, will output a demodulated signal without maintaining intended communication quality.
Even when the intensity of a received signal from the antenna is great or small, for example, a received signal whose modulation system is ASK modulation is input not only to the ASK demodulator but also to the BPSK and QPSK demodulators. That is, the received signal is input to all of the three demodulators. As a result, the two demodulators other than appropriate one will consume electric power wastefully. Even when a signal modulated according to a different modulation system is input, three demodulators consume electric power by demodulation process. However, since the demodulation process cannot produce intended demodulated signals, unnecessary demodulated signals will be output.
Now, consider the problems found in the aforementioned Japanese '554 and '829 publications. The '554 publication, as described above, discloses that demodulated signals are output from the PHS and PDC wave detectors, and one of the two signals is output from the switch according to the signal intensities. In the technique disclosed in the '554 publication, however, the demodulation operation is always performed on the two signals provided from the antenna. As with the aforementioned case, this can cause wasteful power consumption.
On the other hand, the aforementioned '829 publication, as previously described, discloses that the electric power of a received signal from the antenna is measured to determine a modulation system used, and according to the modulation system, either the ASK or QPSK demodulator is selected. In the technique disclosed in the 829 publication, however, in order to determine whether the received signal from the antenna is an ASK or QPSK modulated wave, a variation in power value or amplitude of the received signal is monitored during a period of time equal to or more than one period of the base band signal.
Accordingly, the power consumption of the determiner is considered to be increased. A variation in power value or amplitude of a QPSK modulated wave is small, so that noise, when involved in the QPSK modulated wave, renders the configuration disclosed in the '829 publication to present a strong possibility that the modulation system of a received signal will be determined in error. For example, a received signal will erroneously be switched over to the ASK demodulator, rather than to the QPSK demodulator. Such a possibility of the modulation system being determined in error will take place without regard to the magnitude of the electric power value, or absolute value of the amplitude, of a received signal.